As an alternative to more invasive techniques, many medical procedures are now being performed with minimally invasive catheters or endoscopes. Such devices generally comprise an elongated shaft that is directed to a point of interest. The devices allow a physician to perform a desired task such as taking biopsy samples, performing a therapeutic procedure, or viewing the point of interest from a position outside the body of the patient.
Most catheters and endoscopes have a shaft that is flexible enough to navigate the bends of the human anatomy. The shaft often includes an outer sheath, a reinforcing braid, and a spiral wrap. The spiral wrap increases the column strength of the shaft as well as the crush and kink resistance of the shaft without adding substantially to the stiffness of the shaft. A spiral wrap can be viewed as a helical strip of material separated by gaps. An individual wrap is the portion of the helix making one complete revolution of 360° around the longitudinal axis of the helix. Each wrap has a leading surface edge and a trailing surface edge.
The conventional material used for a spiral wrap is metal. However, metal spiral wraps are expensive to produce because flat metal strips cannot easily be processed into a helix. Furthermore, the thin wall and wide gaps of the metal spiral make it a poor substrate for further processing. Plastics can be used for a spiral wrap, but plastics are more flexible than metals. Therefore, if a plastic is used, the plastic spiral wrap will require a greater wall thickness and more tightly spaced wraps as compared to metal to provide comparable crush and kink resistance.
A disadvantage with plastic spiral wraps is the larger spiral wall thickness combined with the more tightly spaced wraps which causes the surface edges of adjacent wraps to touch as the spiral wrap is bent. As a result of touching, individual wraps push away from each other. This shifts the neutral axis of the shaft when under bending loads and results in axis elongation of the shaft, which, in turn, causes reduced flexibility of the shaft. This reduced flexibility may interfere with accurately tracking a patient's anatomy with the shaft.